Operator Summit Midstream Partners LLC detected the pipeline spill Jan. 6, about 15 miles north of Williston and told health officials then. Officials said they did not get a full account of the size until Tuesday.

Cleanup has begun and inspectors have been monitoring the area, but it will be difficult to measure the effects on the environment and wildlife until the ice melts, said Dave Glatt, chief of the North Dakota Department of Health’s environmental health section. Some previous saltwater spills have taken years to clean up.

At the moment, the spill doesn’t threaten public drinking water or human health, Glatt said. He said they asked a handful of farmers to keep their livestock away from the two creeks, the smaller of which will end up drained.

The saltwater, known as brine, is an unwanted byproduct of oil and natural gas production much saltier than sea water and may also contain petroleum and residue from fracking operations.

The new spill is almost three times larger than one that fouled a portion of the Fort Berthold Indian Reservation in July. Another million-gallon saltwater spill in 2006, near Alexander, is still undergoing clean up nearly a decade later.

Summit Midstream said they pumped out about 65,000 barrels of a mix of freshwater and brine from Blacktail Creek. Brine also reached the bigger Little Muddy Creek and potentially the Missouri River.

Glatt said they will have to fully drain Blacktail Creek as part of the initial cleanup, but the water and soil will have to undergo continuous testing until after the spring thaw because some of the contaminated water froze. They will not drain Little Muddy Creek because it is bigger than Blacktail Creek and because of that there is more of a dilution process going on.

“We will be monitoring to see how quickly it gets back to natural background water quality conditions, and we are already starting to see that,” Glatt said of the Little Muddy Creek. “It’s getting back pretty quickly.”

Summit Midstream’s chief operating officer, Rene Casadaban, said in a statement that the company’s “full and undivided attention” will focus on cleaning up the spill and repairing any environmental damage.

Spokesman Jonathan Morgan did not immediately confirm exactly when the spill began. It also was not clear what caused the pipeline to rupture. Glatt said the company found the damaged portion of pipeline and they sent it to a laboratory to determine what caused the hole.

North Dakota has suffered scores of saltwater spills since the state’s oil boom began in earnest in 2006.

A network of saltwater pipelines extends to hundreds of disposal wells in the western part of the state, where the briny water pumps underground for permanent storage. Legislation to mandate flow meters and cutoff switches on saltwater pipelines ended up overwhelmingly rejected in the Legislature in 2013.

Wayde Schafer, a North Dakota spokesman for the Sierra Club, called the brine “a real toxic mix” and “an extreme threat to the environment and people’s health.”

“Technology exists to prevent these spills and nothing is being done,” said Schafer. “Better pipelines, flow meters, cutoff switches, more inspectors — something has got to be done.”